Method and device for spectral filtering

ABSTRACT

A method and a device in optical fiber based spectral filtering. A length of an optical fiber including at least a core region surrounded by a cladding region is coiled over its length in whole or in part to subject the fiber to longitudinal curvature in order to affect the optical transmission properties of the fiber. The fiber is arranged to have radially asymmetric refractive index distribution and in addition to coiling the fiber lengthwise, the fiber is over its length in whole or in part also twisted around its longitudinal axis. The method and device can be used to significantly improve the performance of fiber based filtering devices.

FIELD OF THE INVENTION

The present invention relates to a method in optical fiber basedspectral filtering. The invention further relates to a spectral filterdevice implementing the method.

BACKGROUND OF THE INVENTION

Telecommunications based on optical fibers is a rapidly evolvingtechnical field. In addition to long distance transmission fibersreplacing more traditional conducting wire cables, a large variety ofother types of optical fiber components are also required in order tomake up a complete modern optical telecommunication system. Suchcomponents include, for example, optical amplifiers based on rare-earthmetal doped active fibers and different types of spectral multiplexingand filtering devices. Spectral filtering in various forms is especiallyimportant in systems based on wavelength division multiplexing, WDM.

It is known that an optical fiber, more specifically a single mode fibercan be used as a spectral filter device by coiling the fiber around areel or corresponding circular body in order to subject the fiberlengthwise to a certain continuous curvature. The radius of thiscurvature determines the cut-off wavelength of such a coiled fiberfilter. With smaller radius of the curvature the cut-off wavelengthmoves towards shorter wavelengths. When the wavelength of the lighttransmitted through the fiber core exceeds the aforementioned cut-offwavelength, the optical power starts to leak from the fiber core to thecladding layer surrounding the core. In the cladding layer the lightexperiences significantly higher attenuation than in the fiber core. Theoverall attenuation characteristics of a the fiber filter can becontrolled by the number of fiber turns coiled around the reel.

In practise, the operation of a coiled fiber filter deviates from idealbecause above the cut-off wavelength all of the wavelengths do notbecome attenuated equally and homogeneously. Because only a finitenumber of guided modes exits in the cladding layer, some wavelengthsbecome coupled to cladding modes more effectively than others. Thelimited number of cladding modes gives rise to a certain amount ofunwanted coupling of the light from the cladding layer back to the fibercore, i.e. reversed coupling effects. As a result of theseaforementioned effects, the typical transmission of a prior art coiledfiber filter shown in FIG. 1 as graph P is not a smooth downward curveafter the cut-off wavelength λ_(off), but instead shows significant“interference” peaks at certain wavelengths. For comparison, FIG. 1 alsoshows a more desirable smooth transmission graph I of a more ideallow-pass filter.

From the prior art certain solutions are known in order to reduce theaforementioned effects. These solutions are primarily based on the ideaof increasing the attenuation of the cladding layer and/or by arrangingthe cladding layer to be surrounded with a specific envelope layer,which allows the light to leak from the cladding layer further to thisoutside envelope or jacket layer. However, these prior art solutionshave certain significant limitations. Because they are basically basedon increasing the attenuation of the cladding layer, they are notsuitable for those applications where also the cladding layer itself isutilized as an optical waveguide. Such applications include, forexample, cladding pumped optical fiber amplifiers, where the pump lightpropagating in the cladding layer should not become attenuated due tothe intrinsic optical properties of the cladding layer.

SUMMARY OF THE INVENTION

The objective of the present invention is to introduce a new approachthat makes it possible to construct optical fiber based spectralfiltering devices, whose spectral properties are superior to the priorart devices. Especially, the intention is to achieve filter deviceswhere after the cut-off wavelength the transmission drops down moresmoothly than in the prior art devices. Further, one specific objectiveof the invention is to construct devices, which are also suitable to beused in the kind of applications, where, in addition to the fiber core,also the cladding layer of the fiber has a role of acting as an opticalwaveguide. An important example of such application can be found amongcladding pumped fiber amplifiers.

In this invention it has been rather surprisingly discovered that theperformance of a coiled optical fiber filter can be significantlyimproved when the fiber is twisted over its length in whole or in partaround its longitudinal axis in addition to subjecting it to a certainlongitudinal curvature.

In order for the longitudinal twisting of the fiber to have the desiredeffect, the optical fiber needs to be of a type without radial symmetry,i.e. radially asymmetrical fiber. Such radially asymmetrical opticalfibers are known as such from the prior art. Radial asymmetry can beachieved, for example, by using an off-centered core, or by providing acladding layer where the refraction index varies in a radiallyasymmetrical manner. Radial asymmetry may also be achieved by usingoptical fiber structures, where the cross-section of the fiber core (oreven cladding) is non-circular. Such fibers are known from polarizationsensitive applications. Fundamentally, in this context the radialasymmetry refers broadly to any optical fiber structures where theradial distribution of the refractive index is asymmetrical.

In an optical fiber filter, where the fiber is both coiled and twistedaccording to the invention, the leak of light from the fiber core to thecladding layer takes place more ideally than in the prior art filters,i.e. without significant amount of reversed coupling effects. Above thecut-off wavelength all wavelengths thus “see” temporally substantiallyequal amount of matching with the cladding modes. In other words, when acertain length of the twisted and coiled optical fiber is considered,with high probability, there always exist such cladding modes whichallow the light to become coupled from the core to the cladding.

As a result of this the transmission curve of the device has a smoothlydescending behaviour after the cut-off wavelength.

The current invention is especially suitable to be used as a distributedspectral filter in cladding pumped fiber amplifiers, because the fiberstructure allows the propagation of the pump light in the claddinglayer.

For a person skilled in the art, it is clear that compared to the priorart solutions, the invention significantly widens the possibilities tooptimize the fiber filter structures. Without “interference” peaks thecut-off wavelength and the attenuation properties of the fiber filtercan be more freely adjusted than in the prior art devices.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows schematically a typical transmission behaviour of a priorart coiled fiber filter together with a more ideal smooth transmissioncurve,

FIG. 2 shows some experimental and comparison results illustrating thebasic transmission properties of an optical fiber filter according tothe invention,

FIGS. 3 a,b describe conceptually the propagation of light in asubstantially straight and radially symmetric fiber,

FIGS. 4 a,b describe conceptually the propagation of light in alongitudinally curved and radially symmetric fiber, and

FIGS. 5 a,b,c describe conceptually the propagation of light in alongitudinally curved and radially asymmetric fiber, which has beentwisted around its longitudinal axis according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following conceptual explanation is meant to describe, in asimplified way, some of the most important physical phenomena behind theinvention. It should be noted that this description is not intended tobe scientifically exhaustive, but it is only meant to help recognise themost essential features of the invention.

To begin, a few measurement results are presented in FIG. 2 in order toillustrate the effect that can be achieved when the optical fiber in acoiled fiber filter is additionally twisted around its longitudinal axisaccording to the invention. It should be understood, that the results inFIG. 2 and the construction of the corresponding fiber filter devicesare provided only to illustrate the effect itself and therefore do notnecessarily correspond with the results or construction of any practicalfiber filter device.

The measurement results shown in FIG. 2 have been recorded using anapproximately 2 meter long single-mode fiber having an off-centeredcore, i.e. a radially asymmetric construction. The fiber core diameterwas 6 μm and the total diameter of the fiber including the claddinglayer was 125 μm. The core was located approximately 30 μm from thecenter. The refractive index distribution of the fiber was of the same“W-type” as schematically shown in FIGS. 5 a–5 c, i.e. with a depressedrefractive index cladding region G next to the fiber core.

In FIG. 2 graph C70 first shows the transmission for the aforementionedoff-centered fiber, which has been coiled without twisting one laparound a reel with an approximately 70 mm diameter. Therefore, graph C70may be regarded to correspond to the performance of a prior art typefiber filter, such as shown schematically in FIG. 1 with graph P. GraphTC70 shows the transmission of the same fiber in an otherwise similarsituation, except that in this case the fiber was twisted around itslongitudinal axis according to the invention. After coiling the fiberwas twisted so that the fiber experienced an approximately 720° twistaround its longitudinal axis substantially evenly along its coiledlength. In other words, the fiber was first coiled one turn around the70 mm reel. Then the fiber was fixed from the starting point of the turnto the reel and the fiber was twisted approximately two full turns fromthe point close to the ending point of the lap. It can be clearly seen,that graph TC70 corresponds to much more desirable transmissionproperties than graph C70.

For comparison, FIG. 2 also contains additional graphs MS70 and MS150.These graphs correspond to coiled, but non-twisted fibers with reeldiameters of approximately 70 and 150 mm, correspondingly. The coatingof these fibers was stripped off and immersion oil was further used formode stripping, i.e. for elimination of the cladding modes.

In the following, with reference to FIGS. 3 a–5 c, the basic physicalphenomena behind the invention are further explained together with somepossible embodiments of the invention.

FIGS. 3 a,3 b describe conceptually the propagation of light in asubstantially straight and radially symmetric fiber 30 comprising a coreregion CR and a cladding region CL. FIG. 3 a shows in its upper sectionthe refractive index profile R and the corresponding mode field M of thefiber 30. In this case the refractive index profile R includes a narrowdepressed refractive index cladding region G in the cladding next to thefiber core. In this depressed refractive index cladding region G therefractive index is arranged to be somewhat lower than in the otherouter parts of the cladding region CL. Such “W-type” refractive indexprofiles R having a certain depressed region G in the refractive indexaround the fiber core are known as such from the prior art.Respectively, FIG. 3 b shows conceptually in its upper section the coremode propagation constant PCR and the cladding mode propagationconstants PCL depicted with horizontal solid lines.

When the wavelength of the light changes, this affects the core modepropagation constant PCR in a known manner. This effect is depicted inFIG. 3 b with arrow A. The core mode propagation constant PCR dependssubstantially linearly on the wavelength. When the core mode propagationconstant PCR decreases the amount of mode field M in the cladding regionCL increases exponentially. When the wavelength of light increases, thecore mode propagation constant PCR becomes smaller and when the coremode propagation constant PCR and that of the cladding modes PCLcoincide, there exists strong coupling from the core mode to thecladding modes. The amount of the mode field in the cladding region CLgives the coupling coefficient between the core mode and the claddingmodes. If and when the propagation constants are the same for the coremode and a cladding mode, the power starts to go back and forth betweenthese two modes.

FIGS. 4 a,4 b describe in a similar conceptual manner the propagation oflight in a longitudinally curved and radially symmetric fiber 40.Therefore, FIGS. 4 a,4 b describe the basic phenomena covering theoperation of a prior art coiled fiber filter.

From FIGS. 4 a,4 b it can be seen that the curvature of the fiber 40 (tothe left in FIGS. 4 a,4 b and also in FIGS. 5 a–5 c) causes an increasein the refractive index in the outer bend of the fiber 40. Therefore,the refractive index profile R becomes tilted as schematically shown inthe upper sections of FIGS. 4 a,4 b. Correspondingly, the propagationconstants of the modes in the cladding region CL in the outer bendbecome elevated. This lowers the cut-off wavelength for a coiled andcurved fiber.

The “interference” peaks shown in FIGS. 1 and 2 (graphs P and C70,respectively) arise due to the fact that there exists only a finitenumber of propagating modes in the cladding layer CL. Therefore, forcertain wavelengths above the cut-off wavelength the conditions becomesuch, that the light power is able to couple to the cladding layer CL(and back) only at certain occasions when moving along the length of thefiber 40. In other words, when a certain length of the fiber 40 isconsidered, the different wavelengths become treated unequally in whatcomes to the coupling between core CR and cladding CL and to theconsequential loss of the light from core CR.

FIGS. 5 a–5 c now describe conceptually the propagation of light in alongitudinally curved and radially asymmetric fiber 50, which has beenfurther twisted around its longitudinal axis according to the invention.FIGS. 5 a–5 c describe three different situations with a relative twistof approximately 90° between FIGS. 5 a and 5 b, and again the samebetween FIGS. 5 b and 5 c.

Because of the twist of the fiber 50, in different locations along thefiber length, the core mode propagation constant PCR can be found tohave moved compared to the cladding modes PCL. The reason for this isthat when moving along the length of the twisted and coiled fiber 50,the core CR moves into different positions compared to the outer curvedsurface (cladding surface) of the fiber (see lower sections of FIGS. 5a–5 c). This “averages” the coupling between the core mode to a certainset of the cladding modes. Now, above the cut-off wavelengthsubstantially all wavelengths, i.e. substantially all core modepropagation constants PCR, “see” temporally an equal amount of matchingwith the cladding modes PCL. In other words, when a certain length ofthe fiber 50 is considered, such cladding modes which allow the light tobecome coupled from the core to the cladding always exist. As a resultof this, above the cut-off wavelength the transmission of the fiber 50has a smooth descending behaviour without disturbing interference peaks.

In the lower sections of FIGS. 5 a–5 c the hatched area CA depicts thecross-sectional area in which the cladding mode propagation constantsPCL are equal or higher than the core mode propagation constant PCR. Inthose situations the core and cladding modes have possibility to matchand energy can move from the core to the cladding layer.

In order for the longitudinal twisting of the fiber 50 to have thedesired effect, the fiber 50 needs to have a certain degree of radialasymmetry. In the embodiment described in FIGS. 5 a–5 c the radialasymmetry is achieved by using an optical fiber 50 with an off-centeredcore CR. However, the current invention is not limited to suchembodiments, but also other means for providing radial asymmetry of therefractive index distribution may be applied. For example, radialasymmetry in a fiber can be achieved by providing a cladding layer CLwhere the refraction index varies in a radially asymmetrical manner.Radial asymmetry may also be achieved by using such fiber structures,where the cross-section of the fiber core CR and/or the fiber claddingCL is non-circular. Such fibers are known, for example, from certainpolarization sensitive applications where the fiber core is non-circularor cladding pumped fibers where the fiber cladding is non-circular.

It should be noted, that even if the fiber 50 shown in FIGS. 5 a–5 cincludes the depressed refractive index cladding region G in thecladding layer next to the fiber core, this is not an absolute necessityfor a fiber filter according to the invention. Such a structure,however, is preferable in many applications because it makes thefiltering effect sharper.

The current invention is especially suitable to be used as a distributedspectral filter in cladding pumped fiber amplifiers, because the fiberstructure now allows the propagation of the pump light in the claddinglayer. For a person skilled in the art, it is clear that compared to theprior art solutions the invention significantly widens the possibilitiesto optimize the fiber filter structures. Without “interference” peaksthe cut-off wavelength and the attenuation properties of the fiberfilter can be freely engineered and fine-tuned according to therespective needs.

The invention also makes it possible to use very large fiber coredesigns (>10 um), which can handle higher laser powers without problemscreated by non-linear optical phenomena.

An important benefit of the invention is that the fiber filter devicesaccording to the invention are simple to manufacture also in practise.In addition to coiling an optical fiber, the fiber only needs to betwisted around its longitudinal axis either before, during or after thecoiling process. The strength of the effect can be adjusted by selectingthe amount of twisting (degrees or turns) per a certain length of thefiber. The twisting may be arranged to appear evenly along the totallength of the fiber, or to be concentrated only to certain parts of thefiber. In a fiber filter having several laps coiled around a reel, thetwisting may be arranged to distribute over all of the coiled laps oronly to some or one of the coiled laps. Depending on the amount ofradial asymmetry of the fiber, the amount of twisting may be freelyadjusted to accomplish desired transmission properties. These and otherparameters, including the length and the optical properties of thefiber, may be freely selected.

Even though the invention has been shown and described above withrespect to selected types of embodiments, it should be understood thatthese embodiments are only examples and that a person skilled in the artcould construct other fiber filter devices utilizing techniques otherthan those specifically disclosed herein while still remaining withinthe spirit and scope of the present invention. It should, therefore, beunderstood that various omissions and substitutions and changes in theform and detail of the filter devices illustrated, as well as in theoperation of the same, may be made by those skilled in the art withoutdeparting from the spirit of the invention. It is the intention,therefore, to restrict the invention only in the manner indicated by thescope of the claims appended hereto.

1. A method in optical fiber based spectral filtering, where a length ofan optical fiber comprising at least a core region surrounded by acladding region is coiled over its length in whole or in part to subjectthe fiber to longitudinal curvature in order to affect the opticaltransmission properties of the fiber, wherein the fiber is arranged tohave radially asymmetric refractive index distribution and in additionto coiling the fiber lengthwise, the fiber is over its length in wholeor in part also twisted around its longitudinal axis.
 2. The methodaccording to claim 1, wherein the optical fiber is arranged to have aradially off-centered core region.
 3. The method according to claim 1,wherein the optical fiber is arranged to have a non-circular core orcladding region.
 4. The method according to claim 1, wherein the opticalfiber is arranged to have a region of depressed refractive index in thecladding region located in the vicinity of the core region.
 5. Themethod according to claim 1, wherein the optical fiber is a single modefiber.
 6. An optical fiber device for spectral filtering containing alength of optical fiber, the fiber comprising at least a core regionsurrounded by a cladding region, and the fiber being coiled over itslength in whole or in part to subject the fiber to longitudinalcurvature in order to affect the optical transmission properties of thefiber, wherein the fiber is arranged to have radially asymmetricrefractive index distribution and in addition to being coiledlengthwise, the fiber is over its length in whole or in part alsoarranged to be twisted around its longitudinal axis.
 7. The deviceaccording to claim 6, wherein the optical fiber has a radiallyoff-centered core region.
 8. The device according to claim 6, whereinthe optical fiber has a non-circular core or cladding region.
 9. Thedevice according to claim 6, wherein the optical fiber has a region ofdepressed refractive index in the cladding region located in thevicinity of the core region.
 10. The device according to claim 6,wherein the optical fiber is a single mode fiber.
 11. The deviceaccording to claim 6, wherein the optical fiber is a rare-earth metaldoped active fiber.
 12. The device according to claim 11, wherein theoptical fiber constitutes a distributed fiber filter in an optical fiberamplifier.
 13. The device according to claim 12, wherein the opticalfiber constitutes a part of a cladding pumped optical fiber amplifier.